Taxonomy & naming
Hyphessobrycon hasemani was described by the American ichthyologist Henry Weed Fowler in 1913, based on material collected from the Rio Madeira above the Falls of Guajaramirim in the state of Rondônia, Brazil. The original description appeared in the Proceedings of the Academy of Natural Sciences of Philadelphia. Eschmeyer's Catalog of Fishes recognises the valid name as Hyphessobrycon hasemani Fowler, 1913, placing it in the family Characidae.
The species belongs to the large and taxonomically complex genus Hyphessobrycon, which contains well over 150 described species spread across tropical South America and Central America. The genus is paraphyletic under most phylogenetic analyses, and its limits are actively debated; H. hasemani is one of the many members whose detailed relationships within the group remain unresolved. Comparative material referenced in recent systematic work on Rio Madeira congeners (Pastana et al.) includes MZUSP lots of H. hasemani ranging from 0.5–1 in SL, confirming morphological characters used to distinguish it from newly described Madeira basin species.
Morphology
Hyphessobrycon hasemani is a small tetra reaching a maximum reported standard length of approximately 1 in (about 1.5 in total length). It shows the typical Hyphessobrycon body plan: laterally compressed, moderately deep-bodied, with the large eye and terminal mouth characteristic of small characids. The caudal fin is forked and the adipose fin is present, as in all characids.
Detailed colour pattern data from field specimens or live material is sparse in the available literature. Like many members of the genus, it likely displays a silvery lateral body with some degree of dark lateral striping or a caudal peduncle spot, though precise markings for this species are not well documented in the accessible English-language hobby or scientific literature. Sexual dimorphism, if present, has not been characterised in the sources examined.
Habitat
The type locality of Hyphessobrycon hasemani is the Rio Madeira above the Falls of Guajaramirim (approximately 10°47'S, 65°23'W), in what is now the Brazilian state of Rondônia. Distributional records place it across the wider Madeira basin, encompassing the Mamoré, Guaporé, and Beni-Madre de Dios systems in the upper Madeira drainage as well as the middle to lower reaches of the Madeira itself — a basin that spans parts of Brazil, Bolivia, and Peru and represents one of the most species-rich river systems in the Amazon region.
The Madeira basin transitions from Andean piedmont rivers in the upper reaches to broad, sediment-laden lowland channels in the lower Madeira, with the intervening stretch punctuated historically by a series of rapids and waterfalls (now mostly inundated by the Santo Antônio and Jirau hydroelectric reservoirs). Specific habitat preferences for H. hasemani within this large basin have not been described in detail, but small Hyphessobrycon species of this region are generally associated with marginal vegetation, flooded forest edges, and the quieter bays and inlets of the main river channel and its tributaries.
Feeding
No specific dietary studies have been conducted on Hyphessobrycon hasemani. By analogy with closely related and sympatric Hyphessobrycon species in the Madeira basin, it is presumed to be an omnivore consuming small aquatic and terrestrial invertebrates, microcrustaceans, insect larvae, and plant or algal matter as available.
In captivity, small tetras of this genus accept a wide range of prepared foods, including fine-grade flake and micro-pellets, as well as frozen and live offerings such as Artemia nauplii, Daphnia, and small bloodworm. A varied diet supports overall condition and, for species kept in breeding programmes, reproductive success.
Mating
Reproductive behaviour specific to Hyphessobrycon hasemani has not been documented in the literature. The genus as a whole is characterised by egg-scattering open-water spawning with no parental care, and this is the reproductive strategy expected for H. hasemani. Males in breeding condition typically pursue females, driving them into fine-leaved vegetation or other submerged structure where eggs are released and fertilised.
Spawning cues in Hyphessobrycon species often include temperature fluctuation, water softening, and increased feeding. Eggs are small, semi-adhesive, and scattered among plants, substrate, or leaf litter; the adults take no interest in the eggs after spawning and will consume them if given the opportunity.
Breeding
Hyphessobrycon hasemani is not an established aquarium breeding subject, and specific captive-breeding data are not recorded in the accessible literature. Based on the genus norm, eggs scattered during spawning would be adhesive or semi-adhesive, settling among fine-leaved plants or onto the substrate. Incubation at tropical temperatures typically spans 24–48 hours for small Hyphessobrycon, with free-swimming fry emerging a day or two after hatching.
The fry of small tetras are minute at hatching and require infusoria or similarly fine first foods before graduating to newly hatched brine shrimp. Separating eggs or adults after spawning is standard practice to prevent predation. Data on typical clutch size, incubation period, and growth rate for this species specifically are not available.
In the aquarium
Hyphessobrycon hasemani is rarely encountered in the aquarium trade and is not an established hobby species. It has been collected in the course of scientific surveys of the Madeira basin and appears in museum collections, but captive husbandry guidelines specific to this species do not exist in the available literature.
Were it to be maintained in captivity, conditions appropriate to a small Madeira-drainage Hyphessobrycon would be a reasonable starting point: soft to moderately hard, slightly acidic to neutral water in the 75–82 °F range, a well-planted tank with subdued lighting, and a group of at least six to promote natural shoaling behaviour. The Madeira basin has been substantially affected by hydroelectric development, and any specimens encountered in trade should be verified as captive-bred rather than wild-caught where possible.
Conservation
Hyphessobrycon hasemani is listed as Data Deficient (DD) on the IUCN Red List, assessed in March 2021. This designation reflects the absence of sufficient data on population size, trends, and habitat use to make a direct or indirect assessment of extinction risk — not a conclusion that the species is not threatened.
The Madeira basin, where H. hasemani occurs, has experienced significant environmental change in recent decades, most notably the construction of the Jirau and Santo Antônio hydroelectric dams on the lower Madeira, which inundated extensive rapids and surrounding habitat. The cumulative effects of dam construction, deforestation, and agricultural expansion in the basin on small-bodied, potentially habitat-restricted fish species like H. hasemani are not well studied. A targeted reassessment with modern collection data would be needed to evaluate its true conservation status.